Dealloying is widely utilized but is a dangerous corrosion process as well. Here we report an atomistic picture of the initial stages of electrochemical dealloying of the model system Cu(3)Au (111). We illuminate the structural and chemical changes during the early stages of dissolution up to the critical potential, using a unique combination of advanced surface-analytical tools. Scanning tunneling microscopy images indicate an interlayer exchange of topmost surface atoms during initial dealloying, while scanning Auger-electron microscopy data clearly reveal that the surface is fully covered by a continuous Au-rich layer at an early stage. Initiating below this first layer a transformation from stacking-reversed toward substrate-oriented Au surface structures is observed close to the critical potential. We further use the observed structural transitions as a reference process to evaluate the mechanistic changes induced by a thiol-based model-inhibition layer applied to suppress surface diffusion. The initial ultrathin Au layer is stabilized with the intermediate island morphology completely suppressed, along an anodic shift of the breakdown potential. Thiol-modification induces a peculiar surface microstructure in the form of microcracks exhibiting a nanoporous core. On the basis of the presented atomic-scale observations, an interlayer exchange mechanism next to pure surface diffusion becomes obvious which may be controlling the layer thickness and its later change in orientation.
The structure of the fluorapatite ͑100͒ surface in humid ambient N 2 with grazing incidence x-ray diffraction is investigated and compared with results on the same surface in dry ambient conditions. Measurement of specular and nonspecular crystal truncation rods provided atomic scale information about the surface structure and the adsorption sites of the water molecules. In humid environment ͑75% relative humidity͒, a laterally ordered monolayer of four water molecules per unit cell is formed at about 1.8 ͑1͒ Å above the relaxed surface reducing the magnitude of atomic relaxations observed on surface in dry conditions.
In this communication, electrodeposition of Zn from 60-40 mol% ZnCl(2)-1-butyl-3-methylimidazolium chloride (BMIC) ionic liquid on Au substrates has been investigated. For the first time, initial stages of Zn electrocrystallization from BMIC has been studied by in situ X-ray diffraction (XRD) employing synchrotron radiation, which showed an initial epitaxial deposition of Zn and hexagonal Au(1.2)Zn(8.8) phases on Au(111) single crystal substrates. In the later stages of electrodeposition, phase analysis showed a formation of several Zn-Au intermetallics, namely AuZn, AuZn(3), and Au(1.2)Zn(8.8), along with the Zn phase.
We discuss how kinetic effects can be utilized to prepare polar ZnO(0001)-Zn surfaces as very well defined and single-crystalline surfaces by hydroxide stabilization of the polar face via a wet chemical etching process in 3N NaOH. An in situ AFM imaging study of the etching process is presented. In addition, measurement and analyses of grazing incidence X-ray diffraction experiments, reflectivity, and crystal truncation rods (CTRs) of the resulting ZnO(0001) surface structures in both dry and humid atmospheres are discussed. Analysis of the CTRs shows that these surfaces are topographically extremely flat, Zn-terminated, but covered with a defect-containing hydroxide/oxygen adlayer, which is adsorbed at hcp-hollow sites. This result is fully consistent with a stabilization of the polar surface by means of an adlayer of disordered hydroxides, which is adsorbed at hcp positions. Moreover, these studies indicate that the water structure at the solid/liquid interface is ordered within the first few layers, but no evidence for an "icelike" structure was found. Also, the pH-dependent stability of these hydroxide-stabilized ZnO(0001) surfaces within electrolyte solutions was investigated by means of an ex situ LEED approach. Hydroxides effectively stabilize the (0001) surface within a wide range of pH values between 11 and 4. In acidic solutions below pH 3.8, the formation of deep hexagonal etching pits is observed, whereas a crystalline structure with triangular reconstructions evolves between these etching pits. The origin of the hexagonal etching pits is discussed as a result of faster dissolution kinetics at dislocation sites.
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